1. Field of the Invention
This invention relates to a highly flame retardant plasticized polyvinyl chloride compound (PVC) further characterized by an absence of brittleness at low temperatures, and substantial flexibility such as required for PVC formed jackets and insulation for wire and cable products and sheets often used, for example, as roof sheathing upon which shingles or other roof covering is laid.
2. Description of the Related Art
PVC compounds (PVC) are a well known class of thermoplastic polymers which exhibit excellent chemical and corrosion resistance, physical and mechanical strength, and electrical insulative properties. Unplasticized versions of PVC are inherently flame resistant and rigid PVC compounds require only additional antimony trioxide to achieve a high level of flame retardancy. However, when flexible forms of PVC are required, the addition of plasticizers result in increased flammability of PVC. Conventional PVC is also apt to produce excessive smoke when ignited.
Both triaryl and diaryl alkyl phosphate esters have been used to improve the flame retardancy of PVC. High flame retardant demands for more stringent flexible PVC applications, such as outer jackets and insulators for plenum wires and cables, for sheets used in the construction industry, for example, as roof sheathing upon which shingles or other roof coverings are laid, as well as for flexible coatings applied to fabrics, have required improvements in the flame retardant plasticizers incorporated in these compounds. The additional of dialkyl tetrahalophthalates such as dioctyl tetrabromophthalate or di-2-ethylhexyltetrabromophthalate have been able to achieve exceptional thermal stability with exceptional flame retardancy. However, the low temperature flexibility of PVC compounds is compromised with the addition of such compounds.
It is known that flame retardant synergy is exhibited by certain brominated and chlorinated compounds. (By use of the term xe2x80x9cflame retardant synergyxe2x80x9d it is meant that the action of two or more substances achieve an effect of which each is individually incapable.) For example, a paper entitled Bromine-Chlorine Synergy To Flame Retard ABS Resins, by Seunghee Yun and Hyunkoo Kim of Miwon Petrochemical Corporation, presented at the New Developments and Future Trends in Fire Safety On A Global Basis International Conference, March 1997, discusses mixtures of DECHLORANE(trademark) PLUS, a product of Occidental Chemical Corporation, and TBBA (Tetrabromobisphenol A), or FF-680 (Bis(Tribromophenoxy)Ethane) to produce acrylonitrile butadiene styrene (ABS) resins having improved flame retardant characteristics. However, the Yun and Kim results refer to ABS, a rigid plastic used in electrical component housings (e.g. TV cabinets and computer casings). Yun and Kim use the chlorinated substance Dechlorane Plus to increase the rigidity of the resultant ABS. Such rigidity would be a serious flaw in PVC formed jackets and insulation for wire and cable products. Improvement of flame retardant properties, as well as improvement of low temperature flexibility and decreased smoke generation, has not heretofore been known when the type of brominated and chlorinated compounds of the present invention are used in PVC compounds. The principal object of the present invention, therefore, is to provide a highly flame retardant plasticized PVC based on the discovery that improved low temperature flexibility can be achieved by adding to PVC dialkyl tetrahalophthalate mixtures of the type hereafter described.
Dialkyl tetrahalophthalates are produced from the reaction of tetrachlorophthalic acid or anhydride and/or tetrabromophthalic acid or anhydride with C1-C18 alkanols. The production of dialkyl tetrabromophthalate and dialkyl tetrachlorophthalates is well known in the field. Numerous processes have been described for the preparation of dialkyl phthalates by esterification of various alcohols with phthalic anhydride or acid in the presence of acidic catalysts, such as sulfuric acid, phosphoric acid, toluene sulfonic acid, and methane sulfonic acid. For example, Spatz et al. (I and EC Product Res. and Dev. 8: 391, 1969) discloses the preparation of di-2-ethylhexyl tetrabromophthalate using phosphoric acid catalysis. Nomura et al. (published Japanese Patent Application No. 50-05701, 1975) describes the use of tetraalkyl titanates in the presence of alkali metal salt to prepare dialkyl tetrabromophthalates. Sagara et al. (U.S. Pat. No. 4,284,793) discloses a method for producing plasticizers with low residual titanium, in which phthalic anhydride is reacted with an alcohol in the presence of a titanate catalyst. The resultant ester is treated with a solid alkali, such as sodium carbonate, and adsorbing agent(s) in the absence of water. Watanable et al. (U.S. Pat. No. 4,304,925) discloses a process for purifying esters, such as those formed from phthalic anhydride and ethyl hexyl alcohol, when organotitanium compounds are used as catalysts, where water is added to the esterification mixture and the mixture is heated. Mamuzic et al. (U.S. Pat. No. 4,754,053) discloses the preparation of tetrabromophthalate diesters using sodium carbonate decahydrate as an essential part of the process. Bohen et al. (U.S. Pat. Nos. 5,049,697 and 5,208,366) disclosed a process for the preparation of dialkyl esters of polyhaloaromatic acids catalyzed by the use of various organometallic catalysts, such as organotitanates, as well as organo-tin, antimony and zirconium compounds. However, these patents do not disclose nor suggest mixing tetrabromophthalate and tetrachlorophthalate for the purpose of improving low temperature flexibility of PVC.
Accordingly, the primary object of this invention is to improve the low temperature flexibility of highly flame retardant plasticized PVC compounds by incorporating into PVC, according to the discovery of the invention, dialkyl tetrahalophthalate mixtures containing both tetrabromophthalates and tetrachlorophthalates.
A further object of this invention is to decrease the smoke generation character of PVC.
Another object is to provide an improved PVC approved for use in products such as insulation, jackets, coatings, and sheeting.
Other objects and advantages will be more fully apparent from the following disclosure and claims.
The subject of this invention is to improve the low temperature flexibility of highly flame retardant plasticized PVC compounds by incorporating into PVC dialkyl tetrahalophthalate mixtures containing both tetrabromophthalates and tetrachlorophthalates. PVC containing tetrabromophthalate and tetrachlorophthalate mixtures show significant improvements in their low temperature flexibility. PVC compounds of the present invention also result in synergist improvements of flame retardancy with decreased smoke generation.
The principal object of this invention as previously stated is to improve the low temperature flexibility of highly flame retardant plasticized PVC by the addition of dialkyl tetrahalophthalate mixtures. Preferably the dialkyl tetrahalophthalates used in the PVC compound of the present invention are prepared in accordance with the teachings of the separately filed co-pending patent application Ser. No. 08/554,262, which disclosed a new method for preparing nearly colorless, highly pure, tetrahalophthalates with an extremely low acid number. However, it is recognized that the dialkyl tetrahalophthalates used in the PVC compound of the present invention may be prepared using methods already known in the art. When tetrahalophthalate compounds are made according to the method described in the ""262 application, certain improved characteristics of PVC useful to the present inventions are achieved. The method for preparing dialkyl tetrahalophthalates as taught by the ""262 application is utilized by the present invention to produce the brominated and chlorinated compounds which are incorporated into the PVC of the present invention resulting in an improved low temperature flexibility characteristic. This method of the ""262 application is described next.
I. Preparation of Dialkyl Tetrahalophthalates
The preferred method as taught in the ""262 application for preparing dialkyl tetrahalophthalates used in the present invention utilizes a tetrahalophthalic compound selected from the group consisting of tetrahalophthalic anhydrides and tetrahalophthalic acids. The tetrahalophthalic anhydride or acid used may be of tetrabromo- or tetrachloro-substitution on the aromatic ring, with tetrabromophthalic anhydride the preferred embodiment The tetrahalophthalic compound is reacted with an excess of alkanol, as is known in the art, to form a reaction mass. The alkanol is selected from C1 to C18 alkanols, or a mixture thereof. The alkanol may be a C1 to C18 primary or secondary alkanol with linear or branched alkyl moieties. The preferred alkanols are 2-ethylhexanol and 3,3,5-trimethylhexanol, as well as mixtures of C8 to C15 alkanols resultant from Oxo- and Ziegler manufacturing processes as known in the art (see Weissermel, K. and Arpe, H-J., Industrial Organic Chemistry, pages 132-134, 206-208, VCH Publishers, New York, 1978).
The tetrahalophthalic anhydride may contain up to 0.30% residual sulfuric acid. Residual sulfuric acid resultant from the preparation of the original tetrahalophthalic anhydride is removed by serial hot water washing or preferably by the neutralization with a first Group II alkaline earth metal salt, more preferably a Group II alkaline earth metal salt of a lower carbon chain acid, and most preferably magnesium or calcium acetate. This treatment must be done before esterification to achieve low product color. Lower carbon chain acids of the type identified herein are carbon chains containing approximately four or less carbons. If hot water is used (about 90xc2x0 C.), typically 3-4, 300-ml aliquots of water (usually about 1 part water to 2 parts anhydride+alkanol) have been found to be sufficient to remove the residual sulfuric acid in the aqueous phase to less than 0.1%, when a mole of anhydride is using in the starting reaction mix. If less water is used, more washes are generally required to achieve the results of the invention. The water wash treatment, while being effective, is substantially more time-consuming than adding a Group II alkaline earth metal salt, which is the preferred treatment.
Preferably, treatment first with a Group II alkaline earth metal salt (first Group II alkaline earth metal salt) is used instead of a water wash to remove acidity. A weak base is preferred, such as magnesium acetate, to neutralize the acid. Alternatively, calcium acetate may be used. Use of an acetate ensures that the pH will be less than 7.0 which is critical because the titanium catalyst is sensitive to, and is destroyed by, alkaline pH. The first Group II alkaline earth metal salt is used at 0.01 to 10 percent of the weight of the reaction mass (defined as the weight of the original reactants, preferably at 0.1 to 0.5 weight percent, most preferably at stoichiometric levels equal to that of the residual sulfuric acid in the tetrahalophthalic anhydride.
The solution which has been water washed or treated with a first Group II alkaline earth metal salt is dried by azeotroping out the water to a content of less than 0.05% by means known in the art. The product is then esterified with a neutral catalyst which may be an alkyl titanate catalyst or zirconium tetrabutoxide. An alkyl titanate catalyst is normally used in the industry. The esterification catalyst is a C1 to C18 tetraalkyl titanate, preferably a C8 to C18 tetraalkyl titanate, which most preferably is an alkyl identical to that of the alkanol esterified into the product, thereby limiting the preparation of mixed esters due to the transesterification of the alkyl groups of the catalyst into the product.
In another important step, the residual acidity from the reaction is removed by the addition of 0.1 to 20 percent of a second Group II alkaline earth metal salt, such as magnesium silicate (MagnesolR from the Dallas Group) or calcium silicate, plus an equal weight of water. This step is done after esterification; otherwise, the tetrahalo anhydride would be unnecessarily treated, which would consume raw material needlessly. Magnesium silicate makes the salts of titanium insoluble so that more of the turbidity-causing titanium drops out of the solution. Although magnesium or calcium oxide, or magnesium or calcium hydroxide, may be used at an equal weight percent basis as the magnesium silicate, there is a resultant decrease in filtration rates. Use of Group I alkalis, other than lithium hydroxide and lithium silicate is not desirable because of their alkalinity (loss of product yield with poor filtration), and some metal contamination (Lithium and titanium) occurs with the lithium alkalis.
The addition of the water in the step discussed above is essential for the neutralization of the residual acidity by the magnesium silicate. Further, the water neutralzes the residual unreacted monoester intermediate. Water acts as a phase transfer agent. Magnesium silicate is a powder. The reaction conditions are 50-95xc2x0 C., preferably 90xc2x0 C. for 1 to 4 hours after which the water of the neutralization is removed by vacuum distillation at 90-140xc2x0 C. This drying technique is essential to driving the neutralization to completion as well as the formation of a granular precipitant which is easily removed with conventional filtration methods, such as vacuum or pressure filtration. The final product of the invention can be washed again with lithium hydroxide if it is desired to increase the product purity and assure the minimum level of acidity.
The treatment with magnesium silicate decreases product color 1 to 5 Gardner color units (A.O.C.S. Method Td 12-64T) as compared to 5-15 Gardner color units when the treatment with magnesium silicate is not utilized. This treatment essentially removes all residual titanium, thus producing a haze-free product even at low temperatures. These Group II alkaline earth metal silicates are particular advantageous since filter aids, such as diatomaceous earth, are not required and the Group II alkaline earth metal silicate absorbs only small quantities of finished product, thereby improving product yield.
Products prepared according to the described method have a purity of 93-95.5%, with an average of 95-95.5%, whereas prior methodologies at their best have only reached a purity of about 93%.
The Gardner color units of the final product are decreased to about 10 without the water wash or magnesium acetate treatment but with the magnesium silicate (as compared to about 15 without either treatment). The water wash or the magnesium acetate treatment plus the magnesium silicate treatment decreases the Gardner color units to about 1-2.
The features and advantages of the described method of the ""262 application as applied to the present invention will be more clearly understood by reference to the following examples, which are not to be construed as limiting to the present invention.